Method for the catalytic oxidation of volatile organic compounds

ABSTRACT

A catalyst for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and of CO to CO 2 , comprising:  
     a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to A 14 Cu 24 O 41  (I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to B 4 Cu 5 O 10  (II), where B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, lanthanides; or mixtures thereof; and in that it is prepared in a form which has a large specific surface area, preferably larger than 25 m 2 /g;  
     a method for preparing the catalysts; their use in methods for the full oxidation of VOC and of CO to CO 2 ; and the oxidation methods.

FIELD OF THE INVENTION

[0001] The present invention relates to catalysts for the full oxidationof volatile organic compounds (VOC), particularly hydrocarbons, and to amethod for the full oxidation of volatile organic compounds (VOC) byusing said catalysts.

BACKGROUND OF THE INVENTION

[0002] The total combustion of VOC to CO₂ and H₂O becomes necessary inview of the toxicity and environmental impact of most unburnt VOC. Thegoal is to minimize the release of VOC into the atmosphere and theforming of CO, which is in turn a toxic component.

[0003] The catalysts used most for VOC combustion are:

[0004] a) catalysts based on noble metals, which are characterized by ahigh cost but also by excellent performance in terms of VOC conversion,and operate at temperatures from 200 to 450° C. according to thereactivity of the compound;

[0005] b) catalysts based on mixed oxides, typically chromites of copperor of other metals, or barium hexaaluminate, which are characterized bya lower cost but are active in more drastic conditions (temperaturesfrom 400 to 600° C.). This second class of catalysts is also used forcatalytic combustors for power generation units. In this case, theyoperate at temperatures above 900° C.

[0006] The types of catalyst used for the combustion of VOC are not freefrom drawbacks; high cost (for those based on noble metals) and pooractivity (for the second class, accordingly requiring operation athigher temperatures, in conditions in which morphologic or structuraltransformations are facilitated).

SUMMARY OF THE INVENTION

[0007] The aim of the present invention is to eliminate the drawbacks ofknown types of catalyst for the full oxidation of VOC.

[0008] In particular, an object of the present invention is to providecatalysts for catalytic combustion which are characterized by highactivity, high resistance to temperature, extreme operating conditions,low cost and easy production even as composites and thin films.

[0009] Another object of the present invention is to provide catalystsfor VOC oxidation with high selectivity for carbon dioxide CO₂ withrespect to carbon monoxide CO.

[0010] Another object of the present invention is to provide catalystswhich lead to full oxidation of the VOC in stoichiometric andnon-stoichiometric mixtures of VOC and oxygen (oxidizing or reducingconditions), so that the mixture of gases produced due to VOC oxidationcontains no carbon monoxide but contains only carbon dioxide.

[0011] Another object of the present invention is to provide a methodfor the full oxidation of VOC which avoids carbon monoxide removaloperations and the known negative consequences of the presence of carbonmonoxide in the environment.

[0012] Another object is to provide a method for oxidizing VOC to CO₂which utilizes the full potential of the VOC oxidation reaction, withevident energy-related advantages.

[0013] Another object is to provide a method for eliminating carbonmonoxide from gas mixtures that contain it together with oxygen.

BRIEF DESCRIPTION OF THE INVENTION

[0014] This aim, these objects and others which will become apparentfrom the detailed description of the invention are achieved by catalystsaccording to the present invention, which comprise one or morenon-stoichiometric crystalline compounds conventionally referenced byformulas which respectively correspond to:

A₁₄Cu₂₄O₄₁   1)

B₄Cu₅O₁₀ (BCuO₂ is also cited in the literature)   2)

[0015] and by a method for oxidizing VOC and CO to CO₂ according to thepresent invention, which uses the same catalysts.

[0016] In the first of the above cited formulas, A is Sr or a solidsolution of Sr with alkaline-earth metals, alkaline metals, lanthanides;in the second formula, B is Ca or a solid solution of Ca withalkaline-earth metals, alkaline metals, or lanthanides.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Examples of catalysts according to the invention have theapproximate formula

Sr₁₄CU₂₄O₄₁

[0018] or

Ca₄Cu₅O₁₀

[0019] Both of these compounds and their derivatives by substitution arewidely known in the literature (for Ca₄Cu₅O₁₀ (mentioned as CaCuO₂):Roth et al, J Am Ceram Soc, Vol. 72, p. 1545 (1989); for Sr₁₄Cu₂₄O₄₁:McCarron et al, Mat Res Bull, Vol. 23, p. 1355 (1988)), and particularlyfor the compound Sr₁₄Cu₂₄O₄₁ there is a vast body of literatureassociated with its unusual electronic properties. Although it is notpossible to formulate exactly the above components, they areunequivocally distinguished by their chemical-physical properties andparticularly by the powder diffraction profiles, which correspond to theones listed in JCPDS international powder diffraction tables, on cards43-0025 and 46-0054 for the compounds referred to as Sr₁₄Cu₂₄O₄₁ andCaCuO₂, respectively.

[0020] The fixing properties of said compounds and derivatives thereofhave been described in patent application BO98A000593 hereinincorporated by reference. The same patent application describes the useof said compounds to fix gases and gas fixing devices which comprisesaid compounds.

[0021] The inventors have now found that surprisingly said compounds, ifprepared so as to have a large specific surface area, preferably largerthan 25 m²/g (as measured by the BET method), act as catalysts for VOCoxidation. The inventors have found that the reaction for full oxidationof VOC in the presence of the catalysts according to the inventionoccurs with a high conversion of VOC even at low temperatures.

[0022] Moreover, the inventors have found that the catalysts accordingto the present invention allow VOC oxidation (even when the conversionis partial) with total selectivity toward the forming of CO₂ even inconditions in which there is a significant deficit of oxygen withrespect to the stoichiometric mix. The expression “total selectivity” isused to reference the fact that VOC oxidation occurs until only CO₂ andH₂O are obtained. In other words, in conditions of oxygen deficit, whilethe quantity that corresponds stoichiometrically to the quantity ofoxygen that is present is converted into CO₂ and H₂O, the other fractionof VOC remains unconverted.

[0023] The temperature ranges over which the catalytic oxidation processis active depend crucially on the volatile organic compound to beoxidized. Considering methane as the most stable and least easilyoxidizable hydrocarbon, the activation temperatures of the reaction forfull oxidation of methane constitute the upper limit of the activationtemperatures for any VOC: in particular, the activation temperatures ofmethane are from 300° C. to 350° C. and from 350° C. to 400° C. for thecompounds A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀, respectively. The maximum utilizationtemperatures of the catalysts instead correspond to the decompositiontemperatures of the compounds A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀, which areproximate to 1000° C. and 750° C., respectively.

[0024] The methods for full oxidation of VOC according to the presentinvention may be performed in combustion chambers or in reheat chambersor flue gas chambers.

[0025] The catalytic oxidation reaction of the catalysts according tothe present invention occurs on a fixed bed or on a fluid bed.

[0026] The catalysts according to the present invention can be in theform of granules.

[0027] Advantageously, the catalysts according to the present inventioninclude a substrate material. The substrate can be an inert substrate inthe form of a thin film or a composite material. Preferably, thesubstrate material is constituted by porous substrates which are inertwith respect to the above described active materials, such as Al₂O₃,TiO₂, ZrO₂, CeO₂, MgO, on which the active material is deposited byimpregnation with the aqueous solution of soluble salts (for examplenitrates or citrates or acetates or mixtures thereof) of the constituentmetals in the correct stoichiometric ratios.

[0028] The catalysts according to the invention preferably comprise 5 to20% by weight of a non-stoichiometric crystalline compound,conventionally designated by a formula which corresponds to A₁₄Cu₂₄O₄₁(I), where A is Sr or a solid solution of Sr with alkaline-earth metals,alkaline metals, lanthanides; or a non-stoichiometric crystallinecompound conventionally designated by a formula which corresponds toA₄Cu₅O₁₀ (II), where A is Ca or a solid solution of Ca withalkaline-earth metals, alkaline metals, lanthanides; or mixturesthereof.

[0029] A catalyst comprising a non-stoichiometric crystalline compoundconventionally designated by a formula which corresponds to Sr₁₄Cu₂₄O₄₁can be prepared, for example, with a method according to the inventionwhich comprises the steps of:

[0030] a) immersing a pre-dried granular porous substrate material in anaqueous solution with a molar concentration of Sr(NO₃)₂ from 0.23 M to0.93 M and a molar concentration of Cu(NO₃)₂ from 0.39 M to 1.59 M;

[0031] b) drying at a temperature from 80° C. to 120° C.;

[0032] c) holding at a temperature from 650° C. to 750° C. in a gasstream which contains oxygen until complete decomposition of thenitrates occurs.

[0033] A catalyst comprising a non-stoichiometric crystalline compoundconventionally designated by a formula which corresponds to Ca₄Cu₅O₁₀ isprepared with a method according to the invention which comprises thesteps of:

[0034] a) immersing a pre-dried granular porous substrate material in anaqueous solution of Ca(NO₃)₂ and Cu(NO₃)₂ in an equimolar ratio and at amolar concentration from 0.39 M to 1.39 M;

[0035] b) drying at a temperature from 80° C. to 120° C.;

[0036] c) holding at a temperature from 650° C. to 750° C. in a gasstream which contains oxygen until complete decomposition of thenitrates occurs.

[0037] Furthermore, a catalyst comprising a non-stoichiometriccrystalline compound conventionally designated by a formula whichcorresponds to Ca₄Cu₅O₁₀ is prepared with a method according to theinvention which comprises the steps of:

[0038] a) immersing a pre-dried granular porous substrate material in anaqueous solution obtained by dissolving, with the application of heat,CuO and CaCO₃ in nitric acid, so that the molar ratio between thecomponents of the solution is CuO: CaCO₃:HNO₃=1:0.83:3.2; water andcitric acid is added thereto so that the citric acid: Cu molar ratio isfrom 3.5:1 to 4.0:1;

[0039] b) heating in air until combustion of the organic fraction of theabsorbed material is achieved;

[0040] c) thermal treatment for 4 to 24 hours at a temperature from 650to 750° C. in a stream of gas containing oxygen.

[0041] Preferably, the porous material that is used is constituted byAl₂O₃, ZrO₂, CeO₂, TiO₂, MgO.

[0042] However, one should not consider the present invention to belimited to catalysts prepared with the described methods.

EXAMPLES

[0043] The catalysts according to the present invention and theoxidation method according to the present invention are described ingreater detail hereinafter with examples which are given only by way ofnon-limitative illustration.

Examples of Preparation

[0044] 1) After drying at 150° C. for 12 hours, an appropriate amount ofporous alumina with a particle size from 2 to 4 mm, specific surfacearea of approximately 400 m²/cm³ and capable of absorbing at least 1 mlof solution per gram is immersed in an aqueous solution of Sr(NO₃)₂ andof Cu(NO₃)₂, at a concentration of 0.556 M and 0.952 M, respectively.The amount of alumina used should be such as to absorb almost all thesolution. The alumina impregnated by the solution is then dripped, driedat 80° C. for 4 hours, and finally treated at 650° C. in a stream of airor oxygen for 24 h. This method produces a catalyst which contains 12%by weight of Sr₁₄Cu₂₄O₄₁ compound and has a specific surface area inexcess of 200 m²/cm³.

[0045] 2) After drying at 150° C. for 12 hours, an appropriate amount ofporous alumina with a particle size from 2 to 4 mm, specific surfacearea of approximately 400 m²/cm³ and capable of absorbing at least 1 mlof solution per gram is immersed in an aqueous solution of Ca(NO₃)₂ andof Cu(NO₃)₂, in an equimolar ratio and at a concentration of 0.556 M.The amount of alumina used should be such as to absorb almost all thesolution. The alumina impregnated by the solution is then dripped, driedat 80° C. for 4 hours, and finally treated at 650° C. in a stream of airor oxygen for 24 h. This method produces a catalyst which contains 7% byweight of Ca₄Cu₅O₁₀ compound and has a specific surface area in excessof 200 m²/cm³.

[0046] 3) After drying at 150° C. for 12 hours, an appropriate amount ofporous alumina with a particle size from 40 to 80 mesh, specific surfacearea of approximately 400 m²/cm³ and capable of absorbing at least 1 mlof solution per gram is immersed in an aqueous solution obtained bydissolving, with the application of heat (80° C.), one mole of CuO and0.83 moles of CaCO₃ in nitric acid so that the nitric acid/copper oxidemolar ratio is 3.2, water and citric acid being added thereto, in orderto complete dissolution of the reagents, so that the citric acid/Cumolar ratio is 3.8. The amount of alumina used should be such as toabsorb almost all the solution. The alumina impregnated by the solutionis then dripped and heated in air until combustion of the organicfraction of the absorbed material is achieved. The resulting material isthen subjected to thermal treatment at 700° C. in a stream of air oroxygen for 24 h. This method produces a catalyst which contains 11% byweight of Ca₄Cu₅O₁₀ compound and has a specific surface area in excessof 200 m²/cm³.

Example 1

[0047] Sr₁₄Cu₂₄O₄₁: Catalytic Combustion of Methane

[0048] A catalyst containing Sr₁₄Cu₂₄O₄₁ was used in catalytic methanecombustion tests.

[0049] The catalyst was a composite material constituted by an inertporous substrate (Al₂O₃) containing 12% by weight of active compound (60mg).

[0050] The tests were conducted in a quartz fixed-bed microreactor witha diameter of 4 mm, containing 500 mg of catalysts in granules withparticle sizes from 20 to 30 mesh.

[0051] Methane and air were fed to the reactor so that the concentrationof methane in the test was equal to 2% by volume. The tests wereconducted at atmospheric pressure and at a spatial velocity, expressedas GHSV (gas hourly space velocity: hourly volumetric flow-rate infeed/volume or weight of catalyst), equal to 80,000 cm³/gh and 55,000cm³/gh, respectively. The mixture of the reaction products was analyzedby gas chromatography. The only products formed during the tests werecarbon dioxide and water.

[0052] The results of the tests at various spatial velocities were foundto be identical within the experimental error: the result obtained at80,000 cm³/gh is plotted in FIG. 1, where the term “conversion”references the percentage in moles of converted methane with respect tothe moles of supplied methane. The expression “set temperature”designates the temperature of the mixture of gas fed to the reactor, andthe term “measured temperature” designates the temperature of the gasmixture at the outlet of the reactor, which is considered equal to thetemperature of the catalyst.

[0053] Methane conversion begins at a temperature from 300 to 350° C.The resulting conversion curve has an inflection point at thetemperature of 500° C. Total conversion is achieved at 650° C.Throughout the conversion process, the only products obtained were CO₂and H₂O.

[0054] From the point of view of molecule reactivity, methane is to beconsidered difficult to oxidize. The required reaction conditions aretherefore extremely drastic if compared with the other paraffins,olefins or volatile organic compounds. The tests that have beenconducted have shown that the compound according to the invention iscapable of oxidizing methane at relatively low temperatures: this is anindicator of the high full oxidation capacity of the inventioncompounds. As regards the other volatile organic compounds, one shouldassume that total combustion occurs at markedly lower temperatures thanthose found for methane.

Example 2

[0055] Ca₄Cu₅O₁₀: Catalytic Combustion of Methane

[0056] A catalyst containing Ca₄Cu₅O₁₀ was used in catalytic methanecombustion tests. The tests were conducted in a quartz fixed-bedmicroreactor with a diameter of 4 mm, containing 500 mg of catalysts ingranules with particle sizes from 20 to 30 mesh.

[0057] The catalyst was a composite material constituted by an inertporous substrate (Al₂O₃) containing 7.5% by weight of active compound(38 mg).

[0058] Methane and air were fed to the reactor so that the concentrationof methane in the test was equal to 2% by volume. The tests wereconducted at atmospheric pressure and at a spatial velocity, expressedas GHSV (gas hourly space velocity: hourly volumetric flow-rate infeed/volume or weight of catalyst), equal to 80,000 cm³/gh and 55,000cm³/gh, respectively. The mixture of the reaction products was analyzedby gas chromatography. The only products formed during the tests werecarbon dioxide and water.

[0059] The results of the tests at various spatial velocities were foundto be identical within the experimental error: the result obtained at80,000 cm³/gh is plotted in FIG. 2, where the term “conversion”designates the percentage in moles of converted methane with respect tothe moles of supplied methane. The expression “set temperature”designates the temperature of the mixture of gas fed to the reactor, andthe term “measured temperature” designates the temperature of the gasmixture at the outlet of the reactor, which is considered equal to thetemperature of the catalyst.

[0060] Methane conversion begins at a temperature from 350 to 400° C.The resulting conversion curve has an inflection point at thetemperature of 550° C. Total conversion is achieved at 700° C.Throughout the conversion process, the only products obtained were CO₂and H₂O.

Example 3

[0061] Sr₁₄Cu₂₄O₄₁: Tests of Methane Combustion in Oxygen DeficitConditions

[0062] A catalyst Sr₁₄Cu₂₄O₄₁, fully similar to the one used in the testcited in example 1, was used in tests of methane combustion in oxygendeficit conditions. The tests were conducted in a quartz fixed-bedmicroreactor with a diameter of 4 mm, which contained 200 mg of catalystin granules whose dimensions were from 20 to 30 mesh. Methane, oxygenand helium in a proportion of 2:1:20 vol/vol were fed to the reactor.The test was conducted at atmospheric pressure and at a spatialvelocity, expressed as GHSV, equal to 80,000 cm³/gh. The mixture of thereaction products was analyzed by gas chromatography. The productspresent in the gases in output at temperatures below 900° C. were carbondioxide and water, generated in a stoichiometric quantity with respectto the fed oxygen and excess unreacted methane.

[0063] The results of the activity test are plotted in FIG. 3, where theterm “conversion” designates the percentage of moles of convertedmethane with respect to the moles of supplied methane. The amounts ofmethane and oxygen used in the experiment allow a maximum conversion of25% of the methane. The maximum value achieved and plotted in FIG. 3 iswithin the expected experimental error.

Example 4

[0064] Ca₄Cu₅O₁₀: Tests of Methane Combustion in Oxygen Deficit

[0065] A catalyst Ca₄Cu₅O₁₀, fully similar to the one used in the testcited in example 2, was used in tests of methane combustion in oxygendeficit conditions. The tests were conducted in a quartz fixed-bedmicroreactor with a diameter of 4 mm, which contained 500 mg of catalystin granules whose dimensions were from 20 to 30 mesh. Methane, oxygenand helium in a proportion of 2:1:4 vol/vol/vol were fed to the reactor.

[0066] The test was conducted at atmospheric pressure and at a spatialvelocity, expressed as GHSV, equal to 80,000 cm³/gh. The mixture of thereaction products was analyzed by gas chromatography. The productspresent in the gases in output at temperatures below 720° C. were carbondioxide and water, generated in a stoichiometric quantity with respectto the fed oxygen and excess unreacted methane.

[0067] The results of the activity test are plotted in FIG. 4, where theterm “conversion” designates the percentage of moles of convertedmethane with respect to the moles of supplied methane. The amounts ofmethane and oxygen used in the experiment allow a maximum conversion of25% of the methane. The maximum value achieved and plotted in FIG. 4 iswithin the expected experimental error.

Example 5

[0068] Sr₁₄CU₂₄O₄₁: Conversion of CO to CO₂

[0069] The catalyst Sr₁₄Cu₂₄O₄₁ prepared as described in example 1 wasused in tests of conversion of carbon monoxide, CO, to carbon dioxide,CO₂. The tests were conducted in a quartz fixed-bed microreactor with adiameter of 4 mm, containing 0.5 g of catalyst in granules havingdimensions from 20 to 30 mesh. Carbon monoxide, oxygen and nitrogen in avolumetric concentration of 1%, 5% and 94%, respectively, were fed tothe reactor.

[0070] The test was conducted at atmospheric pressure and at a spatialvelocity of 50,000 cm³/hg. The mixture of reaction products was analyzedby gas chromatography, using helium as the carrier gas. The compositionof the resulting mixture of gases was analyzed by measuring theconcentrations of CO and CO₂, which were found to be consistent with thereaction 2CO+O₂→2CO₂.

[0071] The results of the test are plotted in FIG. 5, where the term“conversion” designates the percentage of moles of converted CO withrespect to the moles of supplied CO.

Example 6

[0072] Ca₄Cu₅O₁₀: Conversion of CO to CO₂

[0073] A catalyst containing Ca₄Cu₅O₁₀ was used in tests of conversionof carbon monoxide, CO, to carbon dioxide, CO₂. The tests were conductedin a quartz fixed-bed microreactor with a diameter of 4 mm, containing0.5 g of catalyst in granules having dimensions from 20 to 30 mesh.Carbon monoxide, oxygen and nitrogen in a volumetric concentration of1%, 5% and 94%, respectively, were fed to the reactor.

[0074] The catalyst was a composite material constituted by an inertporous substrate (Al₂O₃) containing 11% by weight of active compound (55mg).

[0075] The test was conducted at atmospheric pressure and at a spatialvelocity of 50,000 cm³/hg. The mixture of reaction products was analyzedby gas chromatography, using helium as the carrier gas. The compositionof the resulting mixture of gases was analyzed by measuring theconcentrations of CO and CO₂, which were found to be consistent with thereaction 2CO+O₂→2CO₂.

[0076] The results of the test are plotted in FIG. 6, where the term“conversion” designates the percentage of moles of converted CO withrespect to the moles of supplied CO.

[0077] The catalysts according to the present invention and the methodfor full oxidation of VOC according to the present invention can be usedwith good results for organic compounds which are gaseous or vaporize atlow temperature, achieving complete combustion at low temperature. TheCO₂ selectivity of the catalysts according to the present invention isan important characteristic. Oxidation of VOC, particularlyhydrocarbons, with catalysts which are not specific for combustionusually leads to the generation of both CO and CO₂. Both products arethermodynamically favored in the conditions that are normally used, andthe ratio is therefore usually conditioned by kinetic factors.

[0078] Accordingly, the specificity of the catalyst in the forming ofCO₂ is linked to the characteristics of the active centers. CO₂ is infact the primary product, as demonstrated by the tests conducted byvarying the contact time, which show that the conversion does not dependon the spatial velocity, and therefore does not derive from theintermediate forming of CO. It is also important to note that theprimary carbon monoxide, i.e., the carbon monoxide that does not derivefrom partial oxidation of VOC, is converted efficiently into carbondioxide at temperatures above 100° C. in the case of A₁₄Cu₂₄O₄₁ and 150°C. in the case of Ca₄Cu₅O₁₀ in the presence of oxygen at a concentrationwhich is sufficient to allow the reaction.

[0079] The catalysts according to the present invention and the methodfor full oxidation of gaseous organic compounds can be used to reduce oreliminate many noxious components of combustion gases generated in anymanner, for example from very large-scale electric power stations downto small combustors for home use, including the important applicationsin the field of engines and of vehicles with internal-combustionengines.

[0080] The selectivity of the CO₂ yield furthermore makes itparticularly interesting to use the catalysts according to the presentinvention in burners for closed environments (gas stoves, water heatersfor sanitary use, cooking equipment, et cetera), in which the presenceof CO and in combustion products is notoriously a severe health hazard.

[0081] Furthermore, the catalysts according to the present invention,particularly the compounds of the Sr₁₄Cu₂₄O₄₁ type, convert the carbonmonoxide that is present in gas mixtures containing oxygen into carbondioxide in a very efficient way and at low temperature.

[0082] The catalysts for catalytic oxidation according to the presentinvention are characterized by high activity, comparable to thatexhibited by the more expensive noble metals and by the materialBa₂Cu₃O₆, by high resistance to temperature and to extreme operatingconditions, and by low cost and simple production even in the form ofcompounds and thin films.

[0083] Furthermore, like the material Ba₂Cu₃O₆, they have a selectivityfor carbon dioxide, CO₂, with respect to carbon monoxide, CO. Theadvantages of the materials A₁₄Cu₂₄O₄₁ and B₄Cu₅O₁₀ with respect to thematerial Ba₂Cu₃O₆ relate to:

[0084] reduced limitation in the deposition of large quantities ofactive material of the supported catalyst containing Ca or Sr withrespect to the catalyst containing Ba;

[0085] a very advantageous cost with respect to Ba compounds;

[0086] lack of toxicity of Ca and Sr with respect to heavy metals suchas Ba;

[0087] exclusively for the compound Sr₁₄Cu₂₄O₄₁, higher catalyticactivity in all full oxidation reactions for equal temperatureconditions.

[0088] The catalysts according to the present invention lead to fulloxidation of VOC in stoichiometric mixtures of methane and oxygen or inexcess of oxygen (oxidizing conditions). In this manner, the mixture ofgases produced by VOC oxidation contains no carbon monoxide but containsonly carbon dioxide. In this manner, the operations for removing carbonmonoxide, and the known negative consequences of its presence in theenvironment, are avoided. Moreover, by oxidizing the carbon monoxide tocarbon dioxide, the entire potential of the VOC oxidation reaction isutilized, providing evident energy-related advantages.

[0089] The disclosures in Italian Patent Application No. BO99A000314from which this application claims priority are incorporated herein byreference.

What is claimed is:
 1. A catalyst for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and of CO to CO₂, comprising: a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to A₁₄Cu₂₄O₄₁ (I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to B₄Cu₅O₁₀ (II), where B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, lanthanides; or mixtures thereof; and in that it is prepared in a form which has a large specific surface area, preferably larger than 25 m²/g.
 2. The catalyst according to claim 1, further comprising a substrate material.
 3. The catalyst according to claim 2, wherein the substrate material is a porous inert material.
 4. The catalyst according to claim 3, wherein said porous inert substrate comprises a material chosen from the group constituted by Al₂O₃, ZrO₂, CeO₂, TiO₂, MgO.
 5. The catalyst according to claim 1, in form of granules.
 6. The catalyst according to claim 2, wherein said substrate is an inert substrate in the form of a thin film.
 7. The catalyst according to claim 2, wherein said substrate is a composite material.
 8. The catalyst according to claim 1, comprising 5% to 20% by weight of a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to A₁₄Cu₂₄O₄₁ (I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to B₄Cu₅O₁₀ (II), where B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, lanthanides; or mixtures thereof.
 9. A method for full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, wherein a catalyst according to claims 1 to 8 is used.
 10. A method for converting carbon monoxide to carbon dioxide, wherein a catalyst according to claim 1 is used.
 11. A method for preparing a catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Sr₁₄Cu₂₄O₄₁, comprising the steps of: a) immersing a pre-dried granular porous substrate material in an aqueous solution with a molar concentration of Sr(NO₃)₂ from 0.23 M to 0.93 M and a molar concentration of Cu(NO₃)₂ from 0.39 M to 1.59 M; b) drying at a temperature from 80° C. to 120° C.; c) holding at a temperature from 650° C. to 750° C. in a gas stream which contains oxygen until complete decomposition of the nitrates occurs.
 12. A method for preparing a catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Ca₄Cu₅O₁₀, comprising the steps of: a) immersing a pre-dried granular porous substrate material in an aqueous solution of Ca(NO₃)₂ and Cu(NO₃)₂ in an equimolar ratio and at a molar concentration from 0.39 M to 1.39 M; b) drying at a temperature from 80° C. to 120° C.; c) holding at a temperature from 650° C. to 750° C. in a gas stream which contains oxygen until complete decomposition of the nitrates occurs.
 13. A method for preparing a catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Ca₄Cu₅O₁₀, comprising the steps of: a) immersing a pre-dried granular porous substrate material in an aqueous solution obtained by dissolving, with the application of heat, CuO and CaCO₃ in nitric acid, so that the molar ratio between the components of the solution is CuO:CaCO₃:HNO₃=1:0.83:3.2; water and citric acid being added thereto so that the citric acid: Cu molar ratio is from 3.5:1 to 4.0:1; b) heating in air until combustion of the organic fraction of the absorbed material is achieved; c) thermal treatment for 4 to 24 hours at a temperature from 650 to 750° C. in a stream of gas containing oxygen.
 14. The method according to claim 11, wherein the porous material is constituted by Al₂O₃, ZrO₂, CeO₂, TiO₂, MgO. 